1. Field of the Invention
The invention relates to a small DC (direct current) motor, and more particularly to a small DC motor which is small in size but which has a large torque.
2. Background Art
In the field of small DC motors, there is an increasing tendency to demand motors small in size and light in weight. In DC motors which have cylindrical motor frames having circular cross sections, there is caused a limitation on a relationship between the shape of a field magnet and necessary magnetization characteristics, and in addition, a separate mounting configuration becomes necessary.
In contrast, in DC motors which have motor frames having quadrangular cross sections, necessary magnetization characteristics or magnetic properties which appear as if they matched a sinusoidal magnetization curve can be generated by increasing the thickness of a field magnet at a corner portion of the motor frame, so that the motor frame can be formed into a shape which produces necessary magnetization characteristics. In addition, since the cross section of the motor frame is quadrangular, any of sides of the motor frame is allowed to extend along a mounting surface, which facilitates mounting.
As examples of the DC motors, there exist DC motors disclosed in JP-A-7-059322 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) and JP-A-2000-279885, respectively.
In the example disclosed in JP-A-7-059322, field magnets are integrally molded in an interior of an angularly cylindrical motor frame having a square cross section. The field magnets are magnetized to exhibit four equal magnetic poles which are polarized, respectively, to an N pole and an S pole in an alternating fashion so that centers of the magnetic poles coincide with corner portions of the motor frame. The field magnets are formed into a shape which is angular on an outside and is curved into an arc-shape on an inside thereof. The field magnets are arranged in such a manner as to be in contact with the adjacent field magnets. A rotor is disposed inside the field magnets so arranged.
FIG. 7 is a drawing which illustrates a conventional configuration of field magnets disclosed in JP-A-2000-279885.
In FIG. 7 illustrated in JP-A-2000-279885, a rectangular prism-shaped motor frame comprises an upper motor frame 101 and a lower motor frame (not illustrated) which are made of a metallic material. A plurality of field magnets 102, which are each externally angular and internally circular, are provided on inner walls of the upper motor frame 101. The field magnets 102 are installed, respectively, at four corner portions of the quadrangular upper motor frame 101 and comprise four field magnets 102a, 102b, 102c and 102d which are each formed into a shape which is angular on an outside and is formed into an arc-shape on an inside thereof. The field magnets 102 are disposed adjacent to each other via a minute gap Δg. A rotor (not illustrated) is disposed inside the field magnets 102 so disposed.
In the example of JP-A-2000-279885, the field magnets 102 which are each externally angular and internally curved in an arc are disposed at the corner portions of the upper motor frame. This configuration is preferable in that necessary magnetization characteristics can be obtained. In this configuration, however, not only an air gap necessary for the armature core to rotate smoothly but also the field magnets 102 are interposed between the motor frame and an armature core. Thus, since the field magnets 102 are so interposed, a length from an axial center of the armature core to a radially outer side thereof is reduced by a radial thickness of the field magnet 102. As a result, the effective magnetic flux per slot of the armature core is lowered, leading to a state where torque generated in the armature core does not work effectively. Furthermore, the number of times of winding a winding which can be wound around the armature core is reduced, and torque generated in the armature core is lowered by such an extent.
Also, in the example of JP-A-7-059322, as in the example of JP-A-2000-279885, the field magnets as well as a necessary air gap are interposed between the motor frame and an armature core. Due to this, in the example of JP-A-7-059322, as in the example of JP-A-2000-279885, torque generated in the armature core becomes small.